1. Field of the Invention
The present invention relates to a bearing device and an information recording and reproducing apparatus.
2. Description of the Related Art
Hitherto, an information recording and reproducing apparatus such as a hard disk has been known, which allows a disk (magnetic recording medium) to store various types of information therein and to reproduce the information therefrom. In general, the information recording and reproducing apparatus includes: a head gimbal assembly having a slider that records a signal in the disk and reproduces the signal therefrom; and an arm (rotational member) in which the head gimbal assembly is mounted on a tip end. The arm is made rotatable by a bearing device provided on a base end thereof. The arm is rotationally moved, whereby the slider is moved to a predetermined position of the disk, thus making it possible to record the signal in the disk and to reproduce the signal therefrom.
FIG. 9 is a cross-sectional view of a bearing device 10 according to a conventional technology. The bearing device 10 includes a shaft 20 and a sleeve 40, which are arranged coaxially with each other. The shaft 20 is connected to a cabinet of the information recording and reproducing apparatus, and an arm 8 of the information recording and reproducing apparatus is connected to an outer periphery of the sleeve 40. Between the shaft 20 and the sleeve 40, a pair of roller bearings 30a and 30b are arranged side by side in an axial direction. Outer peripheral surfaces of outer rings 34 of the respective roller bearings 30a and 30b and an inner peripheral surface of the sleeve 40 are fixedly adhered to each other by adhesive.
Mass of the sleeve 40 is large because the sleeve 40 is made of stainless steel. Therefore, a resonant frequency (resonance) between the sleeve 40 and the arm 8, which are rotational portions, is decreased. Accordingly, there is a problem that the resonance is prone to occur at the time when the sleeve 40 and the arm 8 move rotationally, resulting in a decrease of positioning accuracy of the slider mounted on a tip end of the arm 8.
In this connection, JP 2001-155448 A proposes a technology for increasing the resonant frequency in such a manner that a material of the sleeve is changed to aluminum to achieve weight reduction thereof.
However, the aluminum has a larger coefficient of linear expansion than the stainless steel, and accordingly, in the sleeve 40 made of the aluminum, an amount of thermal expansion thereof when a temperature thereof is high is increased. There is a problem that, if the amount of thermal expansion varies among the respective portions of the sleeve 40, distortion occurs in the respective roller bearings 30a and 30b. Such a problem includes deformation of the respective roller bearings 30a and 30b from a perfect circular shape to an oval shape. Following such an occurrence of the distortion, an amount of rotation of the arm 8 with respect to rotation torque is changed, leading to the decrease of the positioning accuracy of the slider.
Further, the adhesive that fixedly adheres the roller bearings 30a and 30b and the sleeve 40 to each other is contracted at the time of being cured. There is a problem that, if an amount of contraction of the adhesive varies among the respective portions thereof, the distortion occurs in the roller bearings 30a and 30b in a similar way to the above.